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P. Liu
flow on the propeller and no interference between the blades. Finally, the total
aerodynamic force on the blades is obtained in the radial direction. Compared
with the momentum theory, the blade element theory deals with the aerodynamic forces acting on the blades. The blade element theory does not account
for the downwash effect produced by the propeller blades, nor does it account
for interference between the blades. In order to improve the blade element
theory, Joukowski and his postgraduate students created the propeller vortex
theory in four papers published between 1912 and 1915, but in order to
simplify the problem, Joukowski assumed that the circulation of attached
vortex is constant along spanwise, the number of blades is infinite, forming
a vortex basket composed of an intermediate vortex belt, a cylindrical vortex
side surface, and an attached vortex chassis. Considering the actual circulation that varies along the spanwise of the blade, the British fluid dynamicist
H.Glauert proposed a theoretical model of the propeller vortex. This method
applies Prandtl’s finite span theory to the aerodynamic design of the propeller.
According to the finite span theory, when the airflow passes a finite span
wing that generates lift, it will change direction, causing the downwash of the
airflow, The downwash angle depends on the lift and span of the wing. The
finite wingspan flow can be seen as a two-dimensional airfoil flow (infinite
wingspan flow) superimposed downwash flow. The effect of the downwash
reduces the angle of attack of the incoming flow. The application of airfoil
theory for propeller blade element analysis uses the lift and drag characteristic data of the airfoil, and avoids the problem of the aspect ratio of the
finite wing. The interference flow is calculated by the momentum theorem,
depending on the number of blades, the spacing, and the aerodynamic forces
acting on each blade. Therefore, the problem of blade interference is also
considered. This theory is also called the standard strip analysis.
The later invented turbofan engine (as shown in Fig. 7.84) became the
main engine of the modern large transport aircraft. Formally, the front of the
engine is a large fan, followed by a high-speed rotating turbine, and the cold
airflow generated by such engine fans is the main contributor to the thrust.
In general, the economical cruising speed of a turbofan engine aircraft can
reach 800–900 km/h, which is much faster than that of a propeller aircraft.
In order to increase the speed and thrust, compared to the turbofan engine,
the front of the turbine is not a low-speed large diameter fan, but a small
diameter medium pressure compressor that forms a turbojet engine (as shown
in Fig. 7.85). The main thrust of this type of engine comes from the jet
behind the turbine. In general, the cruising speed of the aircraft equipped
with this type of engine can be supersonic, such as the Concorde mid-range
supersonic passenger aircraft developed by France and U. K. (This aircraft
P. Liu
flow on the propeller and no interference between the blades. Finally, the total
aerodynamic force on the blades is obtained in the radial direction. Compared
with the momentum theory, the blade element theory deals with the aerodynamic forces acting on the blades. The blade element theory does not account
for the downwash effect produced by the propeller blades, nor does it account
for interference between the blades. In order to improve the blade element
theory, Joukowski and his postgraduate students created the propeller vortex
theory in four papers published between 1912 and 1915, but in order to
simplify the problem, Joukowski assumed that the circulation of attached
vortex is constant along spanwise, the number of blades is infinite, forming
a vortex basket composed of an intermediate vortex belt, a cylindrical vortex
side surface, and an attached vortex chassis. Considering the actual circulation that varies along the spanwise of the blade, the British fluid dynamicist
H.Glauert proposed a theoretical model of the propeller vortex. This method
applies Prandtl’s finite span theory to the aerodynamic design of the propeller.
According to the finite span theory, when the airflow passes a finite span
wing that generates lift, it will change direction, causing the downwash of the
airflow, The downwash angle depends on the lift and span of the wing. The
finite wingspan flow can be seen as a two-dimensional airfoil flow (infinite
wingspan flow) superimposed downwash flow. The effect of the downwash
reduces the angle of attack of the incoming flow. The application of airfoil
theory for propeller blade element analysis uses the lift and drag characteristic data of the airfoil, and avoids the problem of the aspect ratio of the
finite wing. The interference flow is calculated by the momentum theorem,
depending on the number of blades, the spacing, and the aerodynamic forces
acting on each blade. Therefore, the problem of blade interference is also
considered. This theory is also called the standard strip analysis.
The later invented turbofan engine (as shown in Fig. 7.84) became the
main engine of the modern large transport aircraft. Formally, the front of the
engine is a large fan, followed by a high-speed rotating turbine, and the cold
airflow generated by such engine fans is the main contributor to the thrust.
In general, the economical cruising speed of a turbofan engine aircraft can
reach 800–900 km/h, which is much faster than that of a propeller aircraft.
In order to increase the speed and thrust, compared to the turbofan engine,
the front of the turbine is not a low-speed large diameter fan, but a small
diameter medium pressure compressor that forms a turbojet engine (as shown
in Fig. 7.85). The main thrust of this type of engine comes from the jet
behind the turbine. In general, the cruising speed of the aircraft equipped
with this type of engine can be supersonic, such as the Concorde mid-range
supersonic passenger aircraft developed by France and U. K. (This aircraft